This invention relates to electrochemical cells, and in particular to electrochemical cell components and materials useful in the manufacture of electrochemical cell components.
Electrochemical cells may be classified as either electrolysis cells or fuel cells. Electrolysis cells act as hydrogen generators by electrolytically decomposing water to produce hydrogen and oxygen gases. Fuel cells function by electrochemically reacting a fuel gas such as hydrogen with an oxidant gas such as air or oxygen to generate electricity. A preferred type of electrochemical cell is the “proton exchange membrane” cell, wherein the cathode of the cell is separated from the anode by a proton exchange membrane that facilitates the diffusion of ions and/or water between the cathode and anode, but prohibits the diffusion of fuel and oxidant gases.
The typical electrochemical cell includes a number of individual cells arranged in a stack, with the working fluid directed through the cells via input and output conduits formed within the stack structure. The cells within the stack are sequentially arranged, each including a cathode, a proton exchange membrane, and an anode. Membrane electrode assemblies (MEA) for use in fuel cells are well known, being described for example in U.S. Pat. Nos. 5,272,017 and 3,134,697, which are incorporated by reference herein. The MEA for each cell, sandwiched between electrically conductive gas diffusion layers, is placed between a pair of electrically conductive elements or plates which serve as current collectors for the anode/cathode, and which generally contain an array of grooves in the faces thereof for distributing the gaseous reactants (a fuel gas such as H2 and an oxidant gas such as O2 or air) over the surfaces of the anode and cathode. Such plates are described, for example, in U.S. Pat. Nos. 4,988,583, 5,521,018, and 6,261,710B1. The gaseous reactants are usually saturated, typically with water. Each cathode/membrane/anode assembly (hereinafter “membrane electrode assembly”, or “MEA”) is typically supported on both sides by flow fields comprising screen packs. Such flow fields facilitate fluid movement and membrane hydration and provide mechanical support for the MEA.
A plurality of such cells may be stacked together as a fuel cell stack and connected in electrical series. The stack of cells is also called a “fuel cell” in the art. The cells are separated from each other by an impermeable, electrically conductive plate referred to as a bipolar plate. The bipolar plate thus serves as an electrically conductive separator element between two adjacent cells, and generally also has reactant gas distributing grooves on both external faces thereof. In most cases the bipolar plate also has internal passages through which coolant flows to remove heat from the stack. In the electrochemical cell environment, the active areas of the exterior faces of the bipolar plates are in constant contact with often highly corrosive, acidic solutions at elevated temperatures. Moreover, at least one of the electrode faces may be polarized in the presence of pressurized, saturated air or hydrogen. To survive in such an environment, the bipolar plates must be able to withstand these pressures and be highly resistant to corrosion and degradation. In addition to the bipolar plates placed between each fuel cell, end plates may be necessary to contain the fuel cell stack. The end plates must withstand the same corrosive environment as the bipolar plates.
Bipolar plates are often fabricated from graphite or corrosion resistant metals. Graphite is lightweight, corrosion resistant, and electrically conductive but is also quite brittle and thus prone to cracking, and mechanically difficult to handle, thus increasing production costs. Additionally, graphite is porous, making it very difficult to make the very thin, gas-impervious plates that are desirable for low-weight, low-volume fuel cell stacks. The graphite plates must also be operably connected to the other components by seal rings. Typically the seal ring material contains plasticizers and additives that leach out over time and contaminate the catalyst. Catalyst contamination generally halts energy production. Graphite plates also have relatively low heat conductivity, which does not allow heat generated in the cell to be conducted laterally to the edges of the cell by thermal conductivity. Graphite plates must then be further complicated by having coolant liquid channels formed in them.
Corrosion-resistant metals are generally more electrically conductive but these materials typically have low thermal conductivity and, similar to graphite, bipolar plates formed from these materials also require coolant liquid channels.
Accordingly, there is a perceived need in the art for electrically and thermally conductive, low cost components for electrochemical cells, particularly bipolar plates and end plates, with high mechanical integrity and high chemical resistance.